I see a possibility of this idea as a new type of technology if it could be done:
1- Built as a planetary structure, but would not take up a planetary space (because it would theoretically be orbiting the planet). It will give added defense to protect ships at the planet, and against invasions. It should be able to be attack the same as ships at a planet to be taken out, ahead of an invasion.
2- build like a ship unit at a shipyard with the speed of a shipyard for movement, but act like a military starbase for defense AOE purposes in a set area. you could allow it to be upgraded buy constructors as new weapons or sensor tech becomes available at 1/2 a point from the constructor. (It will be able to aid ships in an area, or outright attack enemies, and sense enemy movement, but little else. Also little defense if any and low hit points, to balance it out. If it was a full starbase or ship, why build it, but if it was cheap and easier to upgrade it would be desirable, but it has to have limitations as to not take the place of a starbase. Basicly a balancing act.
Just a couple of thoughts, probably can't be done, or not wanted, but that's my 2 cents worth.